Currently, Local Area Networks (hereinafter referred to as LANs) are generally based on the 10BASE-T, 100BASE-T, and 1000BASE-T protocols, and receive and send signals by using twisted-pair cables. When a Physical Layer Device (hereinafter referred to as PHY) of a sender communicates with a PHY of a receiver, the sender and the receiver both need to send a 10 Mbps link signal or a 100 Mbps signal (hereinafter referred to as a negotiation signal) to the other party. The negotiation signal is used to notify the other party of communication capability of the sender or the receiver, for example, a speed mode and a duplex mode. If a remote PHY is powered off or a twisted-pair cable is disconnected so that a failure that a local PHY cannot receive the negotiation signal occurs, the local PHY may go into a low power consumption mode, and most of signal processing circuits are turned off to save power consumption, which is especially important for an electronic device powered by a battery. If the failure is recovered, that is, when the local PHY can receive the negotiation signal, the local PHY needs to be wakened to enter a normal working mode, and start an auto-negotiation process. Therefore, in order to save the power consumption without influencing normal work, the PHY requires a signal detection circuit to monitor an energy status (for example, a signal amplitude) on the twisted-pair cable for indicating whether the negotiation signal occurs.
In the prior art, a signal detection circuit is generally implemented by adopting separate devices, so that the cost for setting up a whole signal detection circuit is high. Furthermore, a single threshold of a comparator adopted by the conventional signal detection circuit is set, and therefore, detection flexibility of the whole signal detection circuit is poor.